Recently, the standardization of an optical disk with large storage capacity such as a digital video disk has been discussed. In the standard of digital video disk, an optical disk with a substrate thickness of 0.6 mm is used. On the other hand, in the standard of conventional compact disk, an optical disk with a substrate thickness of 1.2 mm is used. Thus, an optical head device which can be used for the reproducing of both the digital video disk and compact disk is desired.
However, in a conventional optical head device, since an objective lens thereof is designed such that it cancels a spherical aberration as to a disk with a predetermined substrate thickness, there remains a spherical aberration as to another disk with a different substrate thickness, therefore resulting in failure in reproduction. Therefore, many optical head devices have been suggested which can be used for the reproducing of both the digital video disk and compact disk with different substrate thicknesses( for example, Japanese patent application laid-open No. 7-65407 (1995)).
FIG. 1 shows a composition of a conventional optical head device. As shown in FIG. 1, around half of emitted light from a semiconductor laser 237 is reflected by a half mirror 238, being transmitted through an aperture 239, entering an optical path control device 240, thereby being divided into a first light and a second light. The first light emitted from the optical path control device 240 is transmitted through a collimator lens 4, entering an objective lens 6 as a collimated light, thereby being focused on a disk 7 which is a first optical disk such as a digital video disk with a substrate thickness of 0.6 mm. On the other hand, the second light emitted from the optical path control device 240 is transmitted through the collimator lens 4, entering the objective lens 6 as a divergent light, thereby being focused on a disk 8 which is a second optical disk such as a compact disk with a substrate thickness of 1.2 mm.
Then, the first and second lights are reflected on the disk 7 and disk 8, respectively, being reversely transmitted through the objective lens 6, collimator lens 4, optical path control device 240 and aperture 239, around its half being transmitted through the half mirror 238, further being transmitted through a concave lens 241, being received by an optical detector 242.
FIGS. 2A and 2B show a composition of the optical path control device 240. The optical path control device 240 is composed of a prism 243 and a prism 244 which are adhered through a polarization separating film 245. The polarization separating film 245 has a function that a P-polarization component of an incident light is all transmitted through and a S-polarization component of the incident light is all reflected. Of the incident light to the optical path control device 240, an incident light 248, which is a P-polarization component to the polarization separating film 245, as shown in FIG. 2A, is all transmitted through the polarization separating film 245, being reflected on reflection films 246, 247 of the prism 244, again being all transmitted through the polarization separating film 245, then being emitted from the prism 243 as the first light. On the other hand, of the incident light to the optical path control device 240, an incident light 249, which is a S-polarization component to the polarization separating film 245, as shown in FIG. 2B, is all reflected on the polarization separating film 245, being emitted from the prism 243 as the second light.
Accordingly, in the conventional optical head device shown in FIG. 1, by the optical path control device 240, effective optical path lengths as to the first and second lights are different from each other, i.e., the effective optical path length from the semiconductor laser 237 to the collimator lens 4 as to the second light is shorter than that as to the first light. Thus, if the effective optical path length from the semiconductor laser 237 to the collimator lens 4 is adjusted such that the first light enters the objective lens 6 as a collimated light, the second light will enter the objective lens 6 as a divergent light. The objective lens 6 has a spherical aberration that cancels a spherical aberration which occurs when a collimated light entering the objective lens 6 is transmitted through a substrate with a thickness of 0.6 mm. Therefore, when the collimated light entering the objective lens 6 is transmitted through a substrate with a thickness of 1.2 mm, there remains a spherical aberration. However, if a divergent light enters the objective lens 6, there occurs a new spherical aberration due to a movement of the image point of the objective lens 6, thereby the remaining spherical aberration when transmitting through the substrate with 1.2 mm thickness being canceled. Thus, if the difference of the effective optical path lengths from the semiconductor laser 237 to the collimator lens 4 as to the first and second lights is suitably set, the first light can be focused on the disk 7 with a substrate thickness of 0.6 mm with no aberration, and the second light can be focused on the disk 8 with a substrate thickness of 1.2 mm with no aberration.
Meanwhile, a numerical aperture of objective lens in the standard of compact disk is to be smaller than that in the standard of digital video disk. In the composition shown in FIG. 1, since the effective optical path lengths as to the first and second lights are different from each other, by providing the aperture 239 in the optical system, the beam diameter of the second light entering the objective lens 6 can be smaller than the beam diameter of the first light entering the objective lens 6 Therefore, the effective numerical aperture as to the second light of the objective lens 6 is smaller than the effective numerical aperture as to the first light of the objective lens 6, thereby the above requirement being satisfied.
FIGS.3A and 3B show another composition of the optical path control device 240 In the optical head device in FIG. 1. As shown in FIGS.3A and 3B, the optical path control device 240 is composed of a prism 10 and a hologram 250 which is formed on the oblique plane of the prism 10. Of the incident light to the optical path control device 240, an incident light 248, which is a P-polarization component to the hologram 250, as shown in FIG. 3A, is reflected and diffracted as a +1st-order diffraction light by the hologram 250, then being emitted from the prism 10 as the second light. On the other hand, of the incident light to the optical path control device 240, an incident light 249, which is a S-polarization component to the hologram 250, as shown in FIG. 3B, is all reflected by the hologram 250, being emitted from the prism 10 as the first light.
FIG. 4 shows a composition of the hologram 250 in FIGS.3A and 3B. The hologram 250 is composed of a polarization separating film 251 and a hologram layer 252 which are formed on the oblique plane of the prism 10. The polarization separating film 251 has a function that a P-polarization component of an incident light is all transmitted through and a S-polarization component of the incident light is all reflected. The hologram 250 functions as a convex surface mirror to a +1st-order diffraction light. Of the incident light to the hologram 250, the incident light 248, which is a P-polarization component to the polarization separating film 251, is all transmitted through the polarization separating film 251, being reflected and diffracted as a +1st-order diffraction light by the hologram layer 252, again being all transmitted through the polarization separating film 251, then being emitted from the hologram 250 as the second light. On the other hand, of the incident light to the hologram 250, the incident light 249, which is a S-polarization component to the polarization separating film 251, is all reflected by the polarization separating film 251, being emitted from the hologram 250 as the first light. As shown in FIG. 4, by forming a step-like section of the hologram layer 252, the diffraction efficiency to a +1st-order diffraction light can be enhanced.
By using the optical path control device 240 as shown in FIGS.3A and 3B, an apparent luminous point to the second light becomes closer to the optical path control device 240 than that to the first light. Therefore, the effective optical path length from the semiconductor laser 237 to the collimator lens 4 as to the second light can be shorter than that as to the first light.
However, the conventional optical head device as shown in FIG. 1 has a first problem that, since the incident light is divided into the first and second lights by the optical path control device 240, the utilization efficiency as to the respective lights is reduced by half. Namely, if the semiconductor laser 237 has the same output power as that of an usual optical head device, the amount of light received by the optical detector 242 is reduced by half of that in the usual optical head device, thereby the signal-to-noise ratio(S/N ratio) of a reproduced signal being reduced. Thus, to make the amount of light received by the optical detector 242 equal to that in the usual optical head device, the output power of the semiconductor laser 237 needs to be two times that in the usual optical head device. If the recording of the disks 7, 8 is performed as well as the reproducing, the output power of the semiconductor laser 237 needs to be further increased, i.e., it is substantially impossible.
Also, the conventional optical head device as shown in FIG. 1 has a second problem, which is related to a wavelength of light emitted from the semiconductor laser 237. in the standard of digital video disk, a wavelength of 635 nm to 650 is used, and, in the standard of compact disk, a wavelength of 785 nm is used. Herein, in order to reproduce both the digital video disk and the compact disk, a semiconductor laser which outputs light with a wavelength of 635 to 650 nm needs to be used since it can give a more reduced diameter of focused spot. As one kind of compact disk, there is a rewritable-type compact disk. Though the rewritable-type compact disk has a high reflection factor more than 70% in 785 nm wavelength, it has a low reflection factor of around 10% in a wavelength of 635 to 650 nm. Therefore, it is impossible for a conventional optical head device which is adapted to a wavelength of 635 to 650 nm to be used in the reproducing of the rewritable-type compact disk.